The present invention relates to an infrared absorbing glass used for electronic image pickup elements, etc., and its fabrication process.
In general, an infrared absorbing filter for cutting light in the infrared range is used with an electronic image pickup element such as a CCD image sensor so as to make correction for relative visibility. This infrared absorbing filer is also used with a medical endoscope so as to prevent laser light from being guided from YAG laser (of around 1,060 nm) and semiconductor laser (of 800 to 900 nm) to the electronic image element.
For instance, when the intensity of YAG laser is high, an interference film comprising several to several tens of laminated layers, each comprising a low refractive index layer such as a silica layer and a high refractive index layer such as a titania layer, is provided on the infrared absorbing filter to cut a substantial portion of YAG laser by interference and cut the remaining laser component by the infrared absorbing filter.
Such an infrared absorbing glass is required to be capable of effectively cut light in the infrared range, e.g., light of 700 nm or higher such as the aforesaid laser and, at the same time, show a suitable spectral distribution in a visible light region of 380 to 700 nm. More illustratively, the infrared absorbing glass has a spectral transmittance of up to 5% at 800 nm near the wavelength used with solid laser, up to 5% at 1,100 nm near the wavelength used with YAG laser, about up to 8% at around 700 nm or a red wavelength relating greatly to the color reproduction of images, at least 60% at around 500 nm relating to brightness, and at least 40% at around 380 nm relating to color reproduction and brightness. In particular, the infrared absorbing glass should preferably have a spectral transmittance of at least 60% at around 380 nm.
In recent years, a thin filter is needed in view of the size reduction of an optical system. The filter thickness is determined depending on the capability of absorbing infrared rays per unit thickness, and so glasses having high infrared absorption capability per unit thickness are needed. The capability of absorbing infrared rays per unit thickness is determined depending on the concentration of an infrared. absorbing component, and so it is possible to achieve a thin filter by obtaining a glass containing such a component at a high concentration. For instance, divalent copper ions are most suitable for the infrared absorbing component, and so a glass containing much CuO is needed.
However, the larger the content of copper, the lower the transmittance at around 400 nm becomes low. This in turn offers two problems, i.e., low light due to a decrease in the quantity of light in a visible region, and a decrease in the quantity of light in a short wavelength region where CCD sensitivity is low. Two such problem cause the S/N ratio to become worse at a short wavelength, and color reproducibility to become worse due to a decrease in the amount of information.
Glasses for infrared cut filters are generally fabricated by a melting method wherein the starting materials are melted at high temperatures. When silica glass is doped with copper by means of the melting method, however, it is required to melt the silica-glass at a high temperature of about 1,500° C. The higher the temperature, the stronger the reducibility of the atmosphere is, and so copper ions change from divalence to stable monovalence in the reducing atmosphere, resulting in a spectral characteristic change.
This in turn causes the infrared absorption capability and visible light transmittance to decrease. In addition, copper is susceptible to coherence and crystallization in the silica glass, and so it is substantially impossible to elevate the concentration of copper. It is thus very difficult to fabricate the desired glass.
The situation being like this, phosphate glass doped with copper is now often used for infrared absorbing filters. However, a problem with the phosphate glass is that its chemical resistance is low. As disclosed in JP-A's 62-128943 and 4-214043, it is proposed to dope the phosphate glass with aluminum, fluorine, etc., thereby improving their resistance. However, even such doped phosphate glass is still insufficient in applications such as endoscopes where it is required to have high resistance with respect to autoclaves, and washing with acids and alkalis. When an infrared absorbing glass in the form of an infrared cut filter is built in a product, it is often polished into a double-plane glass plate of about 0.1 mm to about 2 mm. The degree of abrasion of glass material is considered as being an index to processability upon glass processing and polishing. A glass material having a degree of abrasion of 200 or greater becomes soft, and cannot be processed due to poor processability. This glass material, if it can somehow be processed, requires some awkward steps. Phosphate glasses, and fluorophosphate glasses have generally large degrees of abrasion, with some glass having a degree of abrasion reaching as high as 400. Thus, these glasses are inferior in processability to, and more sensitive to damage than, ordinary optical glasses having a degree of abrasion of about 50 to 200, offering a problem such as difficulty in thickness control.
When an interference film for cutting YAG laser light, etc. is provided on phosphate or fluorophosphate glass, some problems arise. For instance, stresses occur due to a difference in thermal expansion between both materials because the silica base substance of the interference film component has a coefficient of thermal expansion of the order of 10−7 to 10−6 whereas the coefficient of thermal expansion of the phosphate or fluorophosphate glass is of the order of 10−5 or one order of magnitude greater. When the glass contains much phosphoric acid component, the interference film delaminates due to the absorption of atmospheric moisture.
In view of such problems with the prior art as mentioned above, an object of the invention is to provide an infrared absorbing silica glass which has infrared absorption capability equivalent to or greater than that of a prior art phosphate base infrared absorbing filter and, at the same time, high visible light transmittance as well as excellent chemical resistible and processability that the phosphate base infrared absorbing filter has not, and its fabrication process.